What does the line in files from git mean and where does it come from?

Question

git

Hello.
In some repositories, I sometimes notice this line in cpp files:

// $Id: filename.cpp,v 1.5 2019/10/04 08:11:32 mikem Exp $

I suspect that this is some kind of version control system or xs.

I ask purely for the sake of interest, what is it and what advantages does it give, and why is it placed in the file itself.